Simultaneous measurements of NO and NO2 in the stratosphere leading to an NOx determination have been performed by means of i.r. absorption spectrometry using the Sun as a source in the 5·2 μm band of NO and in the 6·2 μm band of NO2. The observed abundance of NOP peaks at 26 km where it is equal to (4·2 ± 1) × 109 cm-3. The volume mixing ratio of NOp was observed to vary from 1·3 × 10-9 at 20 km to 1·3 × 10-8 at 34 km.